1. Field of the Invention
The present invention relates to an optical rotational position information detecting apparatus mainly used for an apparatus for outputting rotational position signals required as clock signals for a hard disk drive, a photosensitive drum rotational position signal output apparatus for a copying machine, or the like.
2. Related Background Art
FIG. 1A is a plan view of a conventional data writing/reading apparatus used for a data processing apparatus such as a computer. A magnetic disk 2 formed by, for example, coating a disk with a magnetic material is placed on an HDD (Hard Disk Drive) 1 serving as a data writing/reading apparatus. The HDD 1 has a magnetic head arm 4 having a magnetic head slider 3 mounted on its distal end. The magnetic head slider 3 writes an information signal on the magnetic disk 2. A voice coil motor 5 is mounted on the rear end portion of the magnetic head arm 4.
FIG. 1B is a plan view of the magnetic disk 2. FIG. 1C is a view for explaining a servo pattern. A plurality of sectors, each consisting of a servo pattern region and data region, are arranged outside a disk hub 2a of the magnetic disk 2.
In this arrangement, the magnetic disk 2 is set on the HDD 1, and the rotational position of the magnetic head arm 4 is forcibly set to a position corresponding to a desired track by a rotary positioner (not shown). The magnetic head arm 4 is then driven in the track direction to write a servo pattern of an information signal with a resolution 1/2 a data track using the magnetic head slider 3 on the distal end of the arm.
When a data signal is to be written on the circular magnetic disk 2 by using the magnetic head slider 3, a servo track signal as information for positioning the magnetic head slider 3 must be accurately written in advance. For this purpose, a magnetic signal must be written at a desired position on the magnetic disk 2 by accurately detecting position information of the rotational direction of the magnetic disk 2 as well as position information of the magnetic head slider 3 in the track direction which is the radial direction of the magnetic disk 2.
FIG. 1D is a perspective view of the HDD 1 having a magnetic clock head 7 that is used independently of a magnetic head 6 for writing information to accurately detect the rotational direction of the magnetic disk 2. According to this scheme, the magnetic clock head 7 enters the HDD 1 through an opening portion 8 and writes a clock signal of a rotational direction on an outermost peripheral portion of the magnetic disk 2. The rotational position of the magnetic disk 2 is then detected while the clock signal is read by the magnetic clock head 7, and a servo track signal is written on each track using the magnetic head 6 for writing information.
The service life of this magnetic clock head 7 is short because it is used to write clock signals on several ten thousand HDDs 1 in a short period of time. That is, the magnetic clock head 7 itself is a consumable item, and hence maintenance such as replacement is required, resulting in an increase in cost.
In addition, since the gap between the magnetic disk 2 and the magnetic clock head 7 must be kept very small, these members may contact each other for some cause. This structure is therefore structurally undesirable in efficiently and economically mass-producing HDDs 1.
As a means for solving this problem, a laser Doppler scheme of detecting the rotational position information of a rotating object by irradiating it with a laser beam is disclosed in Japanese Patent Application Laid-Open No. 7-29229. According to this laser Doppler scheme, since it is only required to irradiate the disk hub 2a of the magnetic disk 2 with a laser beam, no special part such as a scale needs to be bonded to the magnetic disk 2. In addition, owing to noncontact detection, the detecting unit does not wear.
FIG. 1E is a view showing the arrangement of a laser Doppler velocimeter. This device measures the moving velocity of a moving object by using the Doppler effect that when the moving object is irradiated with a laser beam, the frequency of light scattered by the moving object shifts in proportion to the moving speed. In this device, a laser source 11, collimator lens 12, beam splitter 13, and mirrors 14a and 14b are arranged. An object K to be measured, which moves in the direction indicated by the arrow at a velocity V, is placed in the reflection direction of the two mirrors 14a and 14b, and a condenser lens 15 and photodetector 16 are arranged on the optical path of light reflected by the object K.
In this arrangement, a laser beam emitted from the laser source 11 is collimated into a parallel light beam L1 by the collimator lens 12 and strikes the beam splitter 13 to be split into two light beams L2 and L3. These light beams are reflected by the mirrors 14a and 14b and strike the object K, which is moving at the velocity V, at an incident angle θ. Scattered light from the object K is detected by the photodetector 16 via the condenser lens 15.
The frequency of the scattered light beams originating from the two light beams respectively undergo Doppler shifts +Δf and −Δf. Letting λ be the wavelength of a laser beam, Δf is given byΔf=(V sin θ)/λ  (1)
The scattered light beams having undergone the Doppler shifts +Δf and −Δf interfere with each other to cause brightness changes on the light-receiving surface of the photodetector 16. A frequency F at this time is given byF=2Δf=(2V sin θ)/λ  (2)
If the Doppler frequency F of the photodetector 16 is measured according to equation (2), the velocity V of the object K can be obtained.
When the object K is a rotating object, the velocity V of the object K is given byV=2πrW/60  (3)where r is the irradiation radius and W (rpm) is the rotational velocity.
Equation (2) is finally rewritten intoF=(πrW sin θ)/(15λ)  (4)
If equation (4) is converted into a pulse count N for one revolution, equation (4) is rewritten intoN=(4πr sin θ)/λ  (5)
By detecting this pulse signal, rotational position information can be detected.
(1) It is, however, known that the above conventional optical rotational position information detecting means for detecting rotational position information by using a Doppler signal causes dropouts that are portions in which signal components are statistically omitted. It is therefore difficult to accurately identify a rotational position.
(2) In the above prior art, an NRRO (Non-Repeatable Run Out) corresponding to about 0.1 μm occurs when the magnetic disk 2 of the HDD 1 rotates. To write a stable servo signal, it is very important to form a clock signal while minimizing the influence of this NRRO.